Frequency control apparatus



H. F. WlLDER ETAL FREQUENCY CONTROL APPARATUSv Feb. 3, 1948.v

original Filed June 14,1940 6 sheets-sheet 1 Feb. 3, 194s.` H, F, WlLDER ETAL 2,435,259

" FREQUENCY CONTROL APPARATUS I Original Fil'ed June 14, 1940 6 Sheets-Sheet 2 T o Flqa WEST BOUND .2 9@ INVENTORS 1 HEWILDER ff, 9m AWBREYFOGEL n. BY i Y E 4.1," f I Feb. 3, 1948. H. F. WILDER ETAL 2,435,259'

FREQUENCY CONTROL APPARATUS riginal Filed June 14, 1940 6 Sheets-Sheet 3 Feb. 3, 1948. A H. F. WILDER- ET'AL 2,435,259

FREQUENCY' CONTROL. APPARTUS original Filed June 14, 1940 v 6 sheds-sheet 4 I'l/llz 'F INVENTORS H. Ew! I DER AAMBREYFOGEL T0 FIG. 5

Feb, 3, 1948.

H. F. WILDER ET AL FREQUENCY CONTROL APPARATUS Original Filed June 14, 1940 6 Sheets-Sheet 5 'ro F1s.3

INVENTORS H. F. WI'LDER A.W. BREYFOGEL ATT RNE% high Feb. 3, 11948.

H. F. WILDER ET AL FREQUENCY CONTROL APPARATUS `Original Filed June 14, 1940 ZIO 6 Sheets-Sheet 6 v F l G. v7

ZOl 208 205 A'Eo T lNvENToRs H.EWILDER A.W.BREYFOGEL BY f ATTRNEY Patented Feb. 3,

FREQUENCY CONTROL APPARATUS Harold F. Wilder, Wycko, N. J., and Albert W. Breyfogel, Howard Beach, N. Y., assgnors to The Western Union Telegraph Company, New York, N. Y., a corporation of New York Original application June 14, 1940, Serial No.

Divided and this application September 30, 1942, Serial No. 460,218

6 Claims. (Cl. 25o-36) This invention relates to synchronous communication systems and hasparticular reference to apparatus employed for the repetition of the signals in high speed printing telegraph systems.

The present case is a division of a copending application of H. F. Wilder and A. W. Breyfogel, Serial No. 340,462, led June 14, 1940, and entitled Electronic regenerative repeater, now Patent No. 2,333,281, issued November 2, 1943. The instant invention is disclosed in an improved telegraph repeater of the regenerative type which receives, from one section of a line, signals that may be distorted in magnitude and shifted in phase and transmits them into the next line section at accurately spaced time intervals and free vfrom amplitude distortion.

' t is an inherent characteristic of any regenerative repeater to introduce a small amount of phase shift into the retransmitted signals. This is evident when it is considered that in order to correct the phase shift of incoming signals, some adjustment of the local means for timing the retransmission of the regenerated signals is necessary if the phase shift of the incoming signals is predominantly in one direction for a substantial period of time. Obviously, a change in the freuency of the local timing means effects a change inthe spacing of the intervals at which the signals are' retransmitted. In view of this condition the improved telegraph repeater minimizes, insofar as it is practicable, the amount of phase shift which is introduced into the retransmitted signals.

The phase shift is minimized by providing means for effecting a double integration or scanning of the received signals before they are retransmitted. The effect of such a system is t reduce by the square of the phase shift produced by a single integration or scanning the amount of phase shift present in the retransmitted signals. For example, if the received signal is shifted in phase by five units, a single integration may reduce this by four-fifths, or in other Words, to a phase shift of one unit. If now the resultant signal is scanned a second time, it is possible to reduce the shift of one unit by four-fifths, or in other words, to a phase shift of one-fifth of a unit. Thus, it is seen that the final signal is shifted in phase only one-twenty-ifth of the. amount of phase shift present in the received signal.

In effecting the double integration of the received signals, means are provided for varying independently the frequency iof one alternating current generator so that it may be maintained in phase with the average phase of a relatively small sustained phase shift of the received signals over a relativelylaree period `of time.

The instrumentalities provided for the attainment of these two results insure the correction of each signal which is mutilated by fortuitous disturbances, and at the same time prevent the reflection of individual vcorrections into the retransmitted signals, unless these individual corrections are of such a nature that there appears to be a trend in one direction or the other with reference to a xed frequency.

Another feature of the repeater is the provision of circuits used in conjunction with tubes of the gaseous arc discharge type wherein only the starting of the tubes is controlled by a grid. Heretofore, because it was necessary to negatively bias the tubes below the critical firing potential, the operation of this type of tube has been dependent upon the building up by the incoming signals xof some value lof positive potential to overcome a portion of the negative bias. Such arrangements were therefore eifective only after a reversal by the signals from one polarity to the opposite polarity had actually occurred. This, of course, introduced an undesirable time lag in the response of the tube and at the same time Ioperated the tube when the slope of the incoming signal was at something less than its maximum value.

` At such a time a signal is quite susceptible to the effects of interference, and as a consequence it frequently happened that an incorrect response was obtained by the receiving tubes. The circuits provided for obviating these diiculties cause the receiving tubes to operate exactly at the time of signal reversal, While the slope is at a maximum and the Wave front is the least apt to be distorted by interference.

It is an object of the invention to provide novel means for changing the frequency of an alternating current.

Another object of the instant invention is the in by way of illustration. A regenerative repeater embodying the novel features of the invention will be described in conjunction with the accompanying drawings, of which:

Fig. 1 is a block diagram showing the relation between the various Tu`ni ts' comprising 'thenovel repeater; f

Fig. 2 illustrates the arrangement of Figs. 3, 4,;v

5, and 6;

Fig. 3 shows the signal receiving `and storing portions of the repeater and also the apparatus shifting is of an unpredictable nature and may for eiecting the first integrat'icnior s'canningof t the signals;

Fig. 4 shows the transmitting portionv of the repeater, and also the apparatus for effecting the second integration or scanningof thesignals;

Fig. 5 shows the electronic osciliators and 'some of their associated circuits for obtaining the twc alternating current potentials utilized in there?.

peater;

Fig. 6 shows the remaining apparatus necessary for the generation'of the' alternating current potentials, and also the apparatus employed to vary the frequency of the alternating current gener# ators;

Fig. 7 illustrates in simplified form the principles of the frequency changing' or modulating apparatus; and

Figs. 8 and 9 are vector diagrams of the voltages and currents present in the circuits of Fig. 7.

The repeater embodying the invention will first be described in a generalmanner without reference to any* of 'the details'. Accordingly, reference is made toFigg'l ofthe drawings which illustrates inschematic form the 'inter-relation among the various units of the repeater and 'also those employedto control the operation thereof. The path traversed by Vthe signals in passing through the repeater may be'traced by the heavy solid lines in the direction indicated by the arrows. The incoming signals which may be distorted both as to amplitude" and phase are received by a signal receiver IIL At suitable intervals 'corresponding ap'proxini'atelyY to twice the highest signal reversal'fre'quency a ldevice which in effect is'a switch'- II is `close'd, thereby repeat ingfthe received signals into a"`sgna1 storer I2. Thestore'd signals are'subsequently repeated, upon the closure of anotherswth-like device i3, toa` signal transmitter iby means of which they are retransmitted into'thenext section of line as substantially undistorted signals.

There is provided a source' of alternating current, termed herein a phasing'frequency generator I 5, which operates at a Afrequency of substantially twice thatfofV the line signal reversals. One function of the phasing frequencyv generator is cause one signal to be retarded in phase and the Afollowing signal may be affected so that it is advanced in phase. Consequently, when only this type of pha`se s hifti ng is present, the phasing frequencycorrectcr Il@ will be constantly operating alternately "tois'peed A,up and to retard the phasto alter the; frequency of the periodic device used,

ing frequency'generator. The average frequency takeniove'r a perioclof time will therefore remain consenti:

iff-however, the periodic device which is employedto time the transmission of the signals at the remote station is toperated at a slightly diierent frequency from,the phasing frequency generator? .the aveiseffrequency. ofv this generator will' not remain constant. but will operate v atl a different frequencyfromjhatat which it origitoftime theretransmission of Lthe. signals from the repeater or,l in oth aryl/erols, the time interval or spacing -between s'ucrxessive closures .of the `sanfter;#use agricola must be Qhanged; VThe ciosures ofv the dei/ice, I3 'are controlled by meansy Of atransmttine frequency. generator. I8 in a of its alternating current/output; The output of able signal is transmitted to an apparatus termed l herein a phasing frequency corrector Il. apparatusv is eiective Vto* alter 1the frequency of the alternating current generated'by Vthe phasf ing frequency generator1in-`such a manner that ThisV th y34,11;emating'current yis brought into. Phase with the received signals'.` The timing of the frequency corrector circuits is such that the. frequency ofv theph'asing frequency generator varied remanner "similarto that of the phasing frequency generator I Ewithrespect to the `switch I I. AlsoL' 'pared as to their phase relationbymeansfof a phase comparator AI9. A disagreementbetween thejtwosnraftirs is detectedby means ofthe.

comparatorgl and an appropriate signal is sent to a transmittingfrequency corrector 20. In this case,` however, the timing of .the corrector appara'tus 20' 'is considerably slower thanthe timing of the'corre'c'tor Il. fByreason of this arrangement. 'individu'a1'-discrepancies between the frequenciesofthe generators. I5 and I8 are not imme'diately employedto alter'the frequency of the difference 'whielif would call for `a slowing vdown 'of the'generaQLlB in eiectcancel oneanother.

producing1 no changein vthe frequency of `generatori. But if tlieeo'rrections indicated by the comparator! Q areof such a nature that the nu'mf berlo'f onejpredominates vover the other, or in 'other words',` if it appears, that there is a trend.,

in oneg s ensejor the other, the'transmitting'frel quencyfcorrector 20' ltheirbecomes. effective to, malgejjtheppropriate change in the frequency.

of the transmitting 'frequency generator.

Thus, 'it' is seen; .that Y uit organizan@ of the instrume'ntalities comprising vthe '.repeater; is eff fective. to regenerate. 'each 'distorted received Lsig 11211.?1'11S1iiilii th Sighele@ @Cfiuaielyffl .spaced intervals. Thje apparatus comprising. than phasing frequency'. generatori l.I 5, .the phasecomf parator i6, gand thelphasingjfrequency. corrector- H is"immefiaiirfesegnivet@ each Small stellv. of like'phase shifts c )ccurringvv in the received sig:v

rmitting frequency generatorv I8, .the phase. comparfa'tr I 9 and. theftransmitting frequency cora, re'cti'ir.Y 20 .isf responsive L.onlyftoa, trend in one A; sense or the i)tlieip'of the received signalswithg reseejfit t' th'tiraesmlitinergiisny. gnesis;

.nal's' while the apparatus comprising Vthe transgl y I8; Such an arrangement provides the benets to be derived from correcting for substantially every phase shift and at the same time mini'- mlzes the variations in frequency of the retransmitted signals. i

For a detailed description of the regenerative repeater embodying the instant invention, reference will be made to Figs. 3, 4, 5 and 6, arranged as indicated in Fig. 2. The repeater shown here in detail is employed to relay signals in one direction over a system which is arranged for duplex operation. The signals are originally transmitted by means of the usual arrangements for transmission employed in a synchronous telegraph system. Such a transmitter is illustrated diagrammatically by the device 2l and is connected by a line W extending from the remote station to the receiving apparatus of the repeater. The signals appear as potentials between the points 22 and 23 of the line terminating impedances 2li. These potentials are used to control the operation of a pair of gaseous arc-discharge tubes 25 and 26 by means of connections made to the respective control grids of these tubes. Space current is supplied to the receiving tubes by a source of potential 2, the positive terminal of which is connected through the winding 23 of a transformer 29 to the mid-point of a symmetrically arranged network, each branch of which contains an inductance 30 and a resistance 3l, and to the respective anodes of the receiving tubes. These tubes are operated alternately in response vto reversals of polarity of they line signals impressed thereon. Since it is a characteristic of this type of tube that an arc discharge initiated between the cathode and anode under the control of the grid continues until the anode-to-cathode potential is reduced below the critical value, a commutating condenser 32 is connected between the anodes of the tubes in a well known manner to extinguish the are in one tube when an are discharge is started in the other tube. It is also a characteristic of this type of arc discharge tube that a discharge is initiated when .the potential of the grid with respect to the cathode becomes less negative than some critical value. In operating tubes of this type in re,- sponse to polarized telegraph signals heretofore it has been necessary to bias the tubes negatively sufficiently to prevent the operation of both of .the tubes, when the signal potentials became zero. Hence, in order to operate one of the tubes it was necessary4 that the signal potential applied to the grid become positive in an amount .suflficient to reduce the negative bias to the critical firing potential. Consequently, the operating time of the tube after the received signal had reversed from one polarity to another varied appreciably because of the variation of the slopes of the wave fronts of different signals and also because, at this time, the signals are more susceptible to the effects of interference than at the time of reversal from one polarity to the other. The repeater embodying this invention is provided with means for operating the receiving tubes at the instant of reversal of the line signals. Hence', their operation then becomes entirely independent of signal amplitudes and is entirely dependent upon the phase of the received signals. This type of operation is secured by providing novel feed-back circuits between the anode Also, both of the transmitting tubesare fering only in the magnitude ofthe feed-back potential as described hereinafter) and the other tubes of a similar type which are used elsewhere inthe repeater are provided with similar feedback circuits but which have different timing characteristics. Accordingly, only one of each of these feed-back circuits will be described in detail. i

There is connectedbetween the anode o `the tube 25,.andpthe negativeterminal of the source of potential 2'! a series connection of a resistance 33, two choke coils 3e and 35, and a resistance 35. condensers 31 and 38 are connected between intermediate points of the respective choke coils 3c and 35 and the negative terminal of the sourceI 27. The condensers are shunted by a series connection of resistances 39, d@ and 4I. A connection is also made` to these shunting resistances at the point common to resistances 39 and 40 to vthe grid of the tube 25. The choke coils and the condensers comprise a delay network of such a character that .when a high positive potential is appliedLbetween the anode and the cathode of the tube 25, the potential which is applied to the grid of this tube by the network remains substantially unchanged for a predeterrmned period of time. At the expiration of this timer the grid `feed-back potential is increased positively substantially instantaneously to a predetermined value. The resistances 33 and 35 are for the purpose of damping oscillations .between the inductive and capacitative elements 'of the network after the predetermined feed-back potential. has been-reached. The resistances 3s, t0, and 4| comprise a potentiometer or voltage divider whereby the predetermined grid potential may be obtained from the optential to which the condensers become charged. The cathodes of the respective tubes are connected through a common resistance 42 to the negative terminal of; the battery 21. Thus, the voltage drop across this resistance produced by the current flowingl through the tube which is conducting serves to bias the grid of the nonconducting tube negatively with respect to its associated cathode. This bias is suflicient to prevent operation of the tube in the `absence-o1 other grid potentials. v

For the purpose of describing the action of the positive feed-back circuit, assume that the tube 25 is conducting land the tube 26 is non-conducting. When a signal ofthe proper polarity is impressed upon the receiving impedances 24, the tube 2Bis rendered conducting and, by means of the extinguishingaction of the commutating condenser 32, the tube 25 is rendered non-conducting. The xed negative bias is applied to the grid of the tube 25 as described. At the same time substantially the full potential of the source 2l is impressed between the anode and the cathode of this tube. Whilegthe tube is conducting, the positive potential applied to the anode is relatively` small and as soon as the arc in this tube is extinguished, the potential applied to the anode is materially increased.4 AThis increased potential is applied to the delay network of the choke coils and condensers and, depending upon the constants of thenetwork, a short time thereafter the condensers Bland 38 will instantaneously charge to the full potential ofthe source 27. However, until this happens-the' positive feed-back potentialwhichis applied to vthegrid of this tube is maintained at a small value approximating zero and the tube is controlled by the negative bias and the signal voltages;Therefore, it will be armeni ait-santo i. ated` that any uctuation oroscillation-of thereccived-signals which` does notf reachfor exceed the potentialnecessary toreduce the-negative bias to thecritical ring value vof 'the tube' 25 is ineffectivelto produce this result'atthi'stime.4 The importance -of this featureis that once aVsig-nal has been' 'received and lis registered by means of the tubes 25 and 26, there is introduced into the system a-time delayduring whichthe receiving tubes el'edisabledsofar-'as small disturbances of the received signa-ls are concerned'. The delay is long enough to normally permit steady state signal conditions to befestablished. When such a condition has been 1 established, the receiving tube 25 which `is''non-conductingat this time is substantially instantaneouslyY conditioned to receive the following signals This conditionris such that a predetermined-positive feed-back potential vvis 'applied-between thegrideand-the cathode ofthis tube which is,- in itself, 'just capablecffovercoming or-neutralizlngthe negative bias applied to this tubeto an extent l sufcientlto render the net bias equal to the 'critical Iiiring potential. However,

since the potential-appearing at the pointv 22 lof the receiving `networkls'negativ'e-with respect to th'ecathodeof the tubef2-5, the tube remains-nonconducting, But, when 'the following signal is received andthe potential at the point 22'With re- 'spe'cttothe -cathode ofthe tubev 25 becomes zero asvit is about to reverse fromnegative to positive, the tubeiscontrolled solely bythe net biasing pcten'tial.- Since this potential at this time is the eriticalnegative firing potential, anarc discharge is Started in the tubel! Sf'It hasbeen demonstrated 'that-such an event -extinguishes the arc'in the tube-26 and thereafter for a predetermined time disables thistube as-far asinput potentials of less-than the valuenecessary to reducethe negative-bias to the critical firing potential are con- 'cerned Therefore;v it -is evident that even though the Ypotential appearing at the -point 22 does not immediately become positive butfinsteadvbecomes slightly negative -withrespectxito the cathode, the tube 25 will remain conducting Aand the tube 2B will remain non-conducting'provided-that this potential does notV become negative enough to producea corresponding positive potential at the point 23 which is equal to erin excess of the potential necessary to overcome thefiixed negative bias to the extentrequired to produce'thecritical grid-to`cathode firing potential .of the tube 26. Such yaV 'signal .is 'frequently encountered in4 practice andls caused by interference effects being superimposed upon the telegraph signals. 'I'hese interferenceeiects arefrequentlyof an oscillatory nature having a relativelyV short duration so that ultimately the potentialreversal which initiated conduction' inv Vthe tube `25 is completed, thereby rendering thepoint 22 positive and the point v23 negative with respect to the cathodes of thereceiving tubes.- The delay which is intro- 'duced into theapplioation of the positive feedback potential to the input circuits of the receiving tubes is suiiicient Jtti-prevent all but the most violent and longest sustainedinterference eifects irom interferingwith the-properv operation of the receiving system.

The nature of the local biasing circuits of the receiving tubes is such that, in the absence of signaling-potentials, the tubes will-oscillate.Y It has been found that optimum results lare Vobtained whenV the biasing-circuits are adjusted to cause the tubes tofoscillate'at'approximately twice 'the maximum signal reversal frequency. f

It should bepointed' out thatthese feed-back 8. circuits arenot absolutely'essential for the satis factoryoperati'on of the repeater. However, they do improve its operation so that-it is posslbleito repeat higher frequency signals which maybe subjected to higher levels of interference, and-for thisreason are included in the preferred form of the invention. Another feed-back arrangement which also has been found to give excellent `results is thatv which is provided for some of thel other gaseous arc discharge tubes employed herein and is described in a subsequent portion of the speciiication.

Condenser-s 43 and 44 are connected as shown to the input circuits of the receiving tubes25 and 26'for the purpose of icy-passing certain high frequency charging currents generatedv by the operation of the westbound transmitted 45 which is employed to send signals to the remote station connected to the line W, The inductances 30 and the resistances 3| connected in the plate circuits ofthe receiving tubes 'are resonant and critically damped at a frequency which is about twice the highest dot telegraph signal reversal frequency. The purpose of this arrangementis to insure that the potential which appears between the anodes of .these tubes shall produce signals which have a substantially square Wave form. In order to accomplish this result it is necessary that the inductances 30 be Wound on separate cores so that there is noinductive interaction between them.

The respective anodes of the receiving tubes are connected to the extreme terminals of a series connection of resistances 46 and 41. Each of the connecting circuits includes an identical network for the purpose of producing a pronounced peak at the beginning of each signal impulse. One of such networks comprises a condenser 48 connected inhshunt with a resistance 49. Thus, the signals which are received from the line are repeated by the receiving tubes 25 and 25 and appear as square-topped potentials impressed across the terminals of resistances 46 and 41.

'I'he terminals of these reslstances are connected respectively to the grids of'a pair of tubes 5l) and'l' through resistances 52 and 53, respectively. The cathodes of these tubes are connected by means of aresistance 54 to the midpoint of the resistances 46 and' 41. For the purpose of thisl description the tubes and 5| will be revferred toas the first piek-up tubes and they may be of the 6Z1 type or any equivalent capable of amplifying signals in the voice frequency range. As the polarity of the received signals reverses, thepolarity of the potential impressed upon the input circuits of the rst pickup tubes reverses so that, for example, when the grid of the tube 'Ell' is positively biased .to render this tube conducting, the grid of the tube 5l is negatively biased,v therebyV preventing this tube from conducting space current. The anodes of the flrst tpiclruptubes are connected tothe terminals of a primary winding of a coupling transformer 562 Conductors 5T are connected to the cathodes of the tubes and to the midpoint of the winding-55 and are periodically energized by means of a locally generated impulse so that space current may be furnished through one of the portions ofthe winding 55 to the output circuit oi whichever one of the rst pick-up tubes is conditionedi for conduction by means-of the relayed signal Onthe average, this impulse is supplied over the conductors 51 at a time which is ap- `fproxin'iately lone-halfv a baud, or signal impulse,

flaterthan 'the received signall reversal detected by the receiving tubes 'i5-'and 2s. This process is known herein as the first signal scanning, and the means for producing the scanning impulses and the manner in which their timing is controlled will be discussed in detail in a subsequent portion of the specification. Suiiice it to say that these impulses are generated at a rate which is substantially equal to twice the highest dot signal reversal frequency.

Each scanning impulse produces an impulse of short duration in the primary winding 55 of the transformer 56. first pick-up tubes is conducting, the polarity of the impulse which is induced in the secondary winding 58 of the coupling transformer will operate one or the other of a pair of storing relays 59 and 60. The input circuits of these tubes are connected to the secondary winding 58 of the coupling transformer in a conventional manner as shown. These tubes should possess high amplication factors and may be of the 621 type or their equivalents. The tubes are equipped with feed-back or inverse biasing circuits by means of which their sustained operation in response to the short impulses derived from the winding 53 is secured. These circuits may be described by'assuming that the tube 59 is conducting and the tube B is non-conducting. If, in these circumstances, an impulse is induced in the winding 58 of the coupling transformer 56 of such a character that the lower terminal of this winding is positive with respect to the upper terminal, the grid of tube 60 will be biased positively with respect to its associated cathode, thereby rendering this tube conducting. The negative potential which is applied at the same time to the grid of the storing tube 59 is enective to render this tube non-contucting. Consequently, the potential which is applied to the anode of this tube is raised to some high positive value approaching the full potential of the space current source El. This high positive potential is connected by means of a feed-back resistance 62 to the grid of the tube 60 which has just been rendered conducting. These circuits are so designed that the positive feed-back potential is supplied to the tube last operated before the operating potential derived from the short transformer impulse has been removed. Thus, it is seen that the tube which is non-conducting furnishes a biasing potential to the conducting tube to maintain these conditions. Hence, it becomes possible to operate the storing tubes alternately in response to impulses of very short duration and of opposite also be of the GZ? type or their equivalents. The

anodes of the second pick-up tubes are connected to the terminals of a primary winding 68 of a coupling transformer 59. In a manner similar to that described for the first pick-up Depending upon which of the tubes, space current for the second pick-up tubes is supplied over conductors ill by the periodic impression thereon of a series of locally generated impulses which are known as the second scanning impulses. These impulses are also generated by means which will be described in greater detail in a subsequent portion of the specification. The frequency of the second scanning impulses is substantially equal to twice the highest lsignal reversal frequency. The timing of one of the second scanning impulses with reference to one of the rst scanning impulses is approximately one-half a baud, or signal impulse, later. Each of the second scanning impulses is employed to generate an impulse inf-the primary winding 68 so that there is produced by induction in the secondary winding 'H a series of impulses correspending in number and polarity to the signals which were originally received from the line W. The impulses which are induced in the secondary winding 1| are utilized to control the operation of a pair of transmitting tubes 12 and 13.

These tubes are of the gaseous arc discharge type similar to the receiving tubes 25 and 26. They are also provided with delayed feed-back circuits similar to those described in connection with the receiving tubes. However, in the case of the transmitting tubes, the amount of the positive feed-back to the grid .of the nonconductive tube is slightly less than the potential necessary to reduce the flxednegative bias to the critical ringpotential of the tube. Hence, in the absence of an externalpositive potential applied to the grid, the non-conductive tube ywill not be operated by means of the biasingpotentials. This method of operation is necessary because the impulses derived from the secondary winding 'H of the transformer 69, are of relatively short duration and are spaced in time from one another. Hence, there are no available signalderived potentials to maintain stable operating conditions of thev tubes between signal reversals as in thecase of the receiving tubes. It should be noted that'there is no'delay in the response of the transmitting tubes 'for the reason that the impulses impressed upon the input circuits have extremely steep wave fronts.

The loutput circuits of the transmitting tubes include, in addition tothe network of inductances and resistances similar to those employed with the receiving tubes, another network comprising a condenser 14 and a resistance 15 connected in series between the respective anodes of the transmitting tubes. The anode of the tube 'i3 is connected to ground. The. anodei of the tube i2 is connected through an inductance l5 to the apex Ti of a network of terminating impedances 18. One terminal of the terminating network is connected to the'lineE and the other terminal through an artificial line 19 to ground. As the transmitting tubes 12 and 13 are operated in accordance with the signals which are to be sent into the succeeding section of the line, the polarity of the potential applied to the apex l'i is alternately reversed, according to the' character of the signal being transmitted. .Theelements it, 15 and 'I comprise what is known in the art as an anti-noise set which hasv for its purpose the suppression of currents Which may cause interference with lines inthe vicinityof the line E. Such a device is well known' and its connection in the circuit is conventional and will not be described in any further detail;

" The apparatus which is remployed to generate the scanning impulses for the repeater will be described by having reference particularly to Figs.

l 5 and 6 of the drawings. As previously described,

` two series of scanning impulses are used and are furnished at a vfrequency which is substantially vtwice the highest signal reversal frequency.

l These impulses are generated by electronic means which for convenience comprise two electronic oscillators operating at relatively high frequencies with reference to the desired frequency and differing from one another in' an amount equal accusato tothe desired `vfrequency; The "outputs of the oscillators are 'combined and theT difference,y Aor i oscillatorsv are' 'susceptible of mutually independ- -en't` variations in order to 'provide the desiredre- Ygenerating 'action rof"A the repeater. if' the two other oscillators arearranged to normally oper- :ate at" the same frequenoy .whereby the two beat frequencies vproduced are' substantially' the same, -the're'is a iriarked"tendencyv` for these two os- -cillators to lock 'togetherand Aoperate at the same ffrequ'ency; It iis Vnecessary to prevent such an l occurrence sincei'the frequency variation 'of one -ofrthese oscillators must 'beindep'en-dent of that of the other. r`To obviatethe locking tendency by yshielding 'the wiring, which `is the 'medium' by which thev undesired .couplingis effected, is 'unnecessarily expensive.' "A better way is to adjust thefrequencies. of gtheoscillators'so that one beat f frequency' is 4.produced yat 1 twice the highest telegraph signaling` :frequency and vthe *other beat `frequency isproducedat'f substantially the highxesttelegraphA signaling frequency; Sincethe latter vbeat frequency is unusable/inthis form, it is y doubled by. means of a'frequencydoubler.

Oneiof :the local sources of oscillations vprovided l'includes van 'electron dischargedevi'cell which is arranged 'so vasito have-anegative resistance characteristic. This fdevice isVv provided with an" electrode 8| 'which `is connected so that a secondary emission 4-of electronstherefrom is produced. A source; of .potential -82 is connected to a series arrangement of fa pluralityof resistances 83,"84;and`85. `Theseresistances comprise a voltage jdivider to which connections to the electrode-s ofzthe itube' 8.0 are'made.-v -A shunted arrangement ofa condenser 816 andan inductance 81 "is connected'fbetween'the*electrode-8| and an intermediate -'point von the voltage divider; The apparatusof thiscircuit `is to control the frequency of the oscillations in awell known manner and is :commonly referred toas a tank c ircuit. 'I'he outputcircuitof the oscillation generator 80is coupled electrostatically by means of a condenser .98 to a vparallel arrangement of a pair ofresistances 89 and 90. :From these resistances are derived the input circuits for a pair of vacuum tubes 9| and 92 which may ber of the `6C8G type or otherequivalents. Theioutput'cirrespectively. j Y

VThere is provided' athird oscillator los, the output of which is ampliedby'means of vacuum tubes' |05 `and |06 in substantially the same manner 'as that described for tlieother two oscillators. Similarly, thejamplied output .of the oscillator |'4isii'npress`ed upon 'the multiple connection of windings |01 and |08, respectively, of the trans- "The condenser elements of the tank circuits 86481, and H2 are variable and are arthe two described extremes.

12 ranged to be controlled by a common frequency adjusting control (not shown). The oscillator 9`|A is thecoznrnon oscillator previously referred Hto, and oscillators 30 and |04 are the variable frequency oscillators. The circuit constants of the three tanks are chosenso that, with the common frequency adjusting control set at one extreme position, the oscillations generated by the oscillator 8o haveafrequency of substantially 614.5 cycles per second, those generated by the oscillator 91 substantially 585 cycles per second and those generated by the oscillator |04 substantially 644 cycles per second. Thus, it is seen that the beat frequency derived from oscillators 89 4and 91 is the difference between these respective frequencies which is `29.5 cycles per second. Similarly, the beat frequency derived from oscillators si and |04 is 59 cycles per second or exactly twice the first beat frequency which is subsequently doubled and utilized together with the second beat frequency at times when the telegraph signaling is effected `at approximately 29.5 cycles per/second. The move- .ment of the frequency adjusting control to its other extreme position decreases the frequency of the oscillators 80 and 91 and increases that of the oscillator |04, so that the respective frequencies become substantially 605, 500 and 710 -cycles per second. Now, it will be seen that the beat frequency derived from the oscillators 80 and 9? is 105 cycles per second-and that-derived from the oscillators 91 and |04 is 210 cycles per second. Again, it will be observed that the twoto-one relation exists between the two beat frequencies. The same relation holds for all positions of the frequency adjusting control between These frequencies are selected initially in accordance with the tele- Vgraph signaling frequency. The values given herein are intended only -as illustrations andare not contemplated as limiting the scope of the invention in any manner. Also, it will be understood by those skilled in the art that, by taking the necessary precautions against coupling, the oscillator 80 may be operated at substantially the same frequencyas the oscillator |04 without cle- `parting from the scope of the invention.

"Each of the transformers associated with the output circuits of the frequency amplifiers is provided with a plurality of secondary windings, the purpose of which will become apparent from subsequent portions of the specification. Secondary winding I3 of transformer 95 is connected in series relation with secondary winding H4 of transformer |02. The terminals of this series connection, when properly poled, are connected respectively to the grid and cathode of an amplifier tube l5. Similarly secondary winding H0 of transformer 95 is arranged in series connection with secondary winding II'I' of transformer |32 and the properly poled terminals of this connection are connected respectively to the grid and cathode of a vacuum tube I8. The re- 'sp'ective anodes of the tubes 5 and H3 are connectedvto the terminals of a winding ||9`of transformer 12e. The midpoint of this transformer winding is connected to the positive terminal of a source of direct current potential |2|. The negative terminal of this source of potential isl connected through a resistance |22 to the cathodes of the amplier tubes ||5 and I8. Thus, it is seen that these tubes are connected in a push-pull arrangement. Also, it is seen that `the input circuits of this push-pull amplier are derived from transformer windings in such a 13 manner that a combination' of the frequencies generated by the oscillators 8s and 91 is lmpressed upon the amplifier. By such an arrangement there appears in the primary winding ||9 of the output transformer |25 two potentials having the frequencies of the oscillators 80 and 91, respectively. These potentials are reproduced by induction in the secondary winding |23 of the transformer |20 and are demodulated by means of a full wave rectifier tube IM. in order to produce a series of uni-directional impulses of varying amplitude having an envelope which varies in frequency in accordance with the difference, or beat, frequency produced by the combination of the frequencies generated by the oscillators 80 and 91. This series of impulses is passed through a iilter network |25 which comprises a plurality of inductive and capacitative reactances connected as shown and which has for its function the filtering out of the high frequencies produced by the oscillators. Thus, the potentials which are applied to the primary winding A|26 of a transformer |21 are alternating in character, have a sinusoidal wave form y and a frequency which is the beat, or difference, frequency derived from the oscillators 80 and 91,

and which is equal to the signal reversal frequency.

These potentials are reproduced by induction in the secondary winding |28 of this transformer and are applied to a frequency doubling arrangement by being impressed upon the input circuits of a pair of vacuum tubes |23 and' |30. These tubes may be of the SES type or their equivalents and are biased so as to having non-linear amplifying characteristics. The output circuits of the amplifier tubes |29 and lati are connected to the primary winding |3| of a transformer |32 in a multiple, or push-push connection as shown. Thus, there appears inthe primary winding the second harmonic of the amplifier input frequency and there is derived from the secondary winding of the transformer |32 a sine wave potential having twice the signal reversal frequency. Before the manner in which this alternating current potential -is employed in the repeater is described it may be well to discuss brieiiy the generation of the second alternating current frequency.

In view of the foregoing description the detailed circuits which are involved in the generation of this second alternating current potential will not be described fully since they are precisely similar to those already described. The respective outputs of the oscillators 91 and are combined by means of the secondary windings of transformers |533 and iii) and applied to the input circuits of a pair of pushpull amplifier tubes 33 and i3d. The connections of these tubes to the secondary transformer windings are similar to the connections described in detail for the input circuits of the push-pull amplifier tubes ||5 and I8. The combination potential of the two oscillators 3'! and |04 is demodulated by a rectifier tube |35 and the high frequency components of the resultant wave are filtered out by means of the iiiter ISS. The beat frequency alternating current potential is impressed upon the input circuits of a pair of amplifier tubes |31 and |38 by means of a cou,- pling transformer |39. Because of the difference between the frequencies of oscillation of the oscillators 91 and HM- previously described, there is impressed upon the primary winding |40 of the transformer |4| an alternating current pof 14 i tential having a sinusoidal form and a frequency substantially equal to twice the signal reversal frequency.

The sinusoidal alterna-ting current voltage of approximately twice the telegraph signal reversal frequency which appears in the secondary winding |42 of the transformerV |32 is applied by means of conductors |43 to the input circuits of a pair of gaseous arc-discharge tubes |44 and |45 (see Fig. 3). Each of these tubes is provided with a feed-back circuit which is somewhat similar to the feed-back circuits associated with the receiving tubes but which has a somewhat different timing characteristic. The# feed-back circuit which is associated with the'v tube |44 comprises a resistance |46 in series with a condenser |41, which combination is connected between the anode of the tube and the negative terminal of the source of potential |48, 'A voltage divider is connected in shunt with thecondenser and comprises resistances |49 and |50 and the upper half of the secondary winding |42 of the transformer |32. A connection is made to the grid of the tube |44 from the junction of resistances |49 and |50 whereby the 'desired' positive potential is applied to the grid. As soon as the tube |44 is rendered non-conducting. the anode is raised to substantially the -full positive potential of the battery |48. Consequently, the condenser |41 begins to charge through the resistance |46 at arate which.

is determined by the value of this resistance. As is well known, the potential of the condenser when plotted against time rises 'substantially' according to an exponential curve until ultimately the potential across the condenser reaches a maximum 'steady state value. A portion of this voltage is selected by means of the voltage divider and ap-` plied to the grid of the tube |44 and is just capable when acting by itself in concert with the fixed negative bias to initiate conduction in the tube. The tube |45 is also .provided with a similar feed-back circuit and the action of these tubes in response to the exciting potentials applied to the conductors |43 is identical with the action of the receiving tubes previously described. In this case, however, it willbe observed that the positive bias whichis applied to the tubes after they become extinguishedis` gradually built up rather than beingapplied instantaneously after a predetermined delay as in the case of the receiving tubes. This type of operation is equally as good as that obtained with the feed-back circuits previouslydescribed.

The output circuits of these tubes also include apparatus similar to that previously described, and in addition-there is included in each of these circuits, in series with the anodes of the respective tubes, primary windings |5| and |52 of transformers |53 and |54, respectively. Since these tubes are driven under the control of potentials havinga |frequency twice the highest signal reversal frequency, there will appear across the terminals of apair of 'resistors |55 'and 55 an alternating current potential of twice the signal reversal frequency. This potential is applied to the input circuits of a pair of vacuum tubes |51 and |53V which may be of the 6Z7 type or their equivalents. In each of the circuits for space current of 'these tubes there is included one-half of the primary winding- |59 of a transformer |50 and the secondary winding |6| of the transformer 29.

For'the description of the function 0f the tubes |51 and |58, let us assume initially that the signais which are being repeated from the -line W into the line E are perfect ones requiring no correctionaby thea repeater; This, of ;,course,' isl :an idealcondition not `practically-1l realizablein -,prac. tice, but it is believed that the invention'fmayvbe better understood; iffthis` ideallsituation` is assumedsto exist. It will betrememberedthat asthe receiving tubes 25 'and 26 lare operated alternately in` response to received, signal reversals, a` surge of current or a transient eiectds produced, in the primary winding 28 of the'transformer 29 `due to '.the action of the cornmutatingv rcondenser 132 These surges are all of the same polarity'and therefore producefbyinductionza series ofunidirectionalimpulses inY the secondary winding |61 -of thisA transformer. Thesesimpulses are of shortqdurationand serve as thesource of J'space current for Vthetubes |51and11|582 Thetiming ofthese impulses'under the ideal-conditions asf sumed is such thattheyoccur at -the precise mo.-

ment that a reversal of theainputfpotentials applied to these ,tubes'is occurring; Hence, forex?- ample;v the Vimpedance of the tube.y |51 'is/,at this time :being increased and A.theyimpedance.. ofthe tube c- 58s is beingfdecreased; Consequently, .the impulse which is generated inxthe-,transforxner Winding |61 will produce -in theseeondary=wind= ing|52 of the transformer: |61a transient which comprisesza half .cycle of vonefpolarityfollowediby a half cycle of the opposite polarity.` Thistransient voltage is impressedupon a pairofconductors |63 which lead to the-correcting.VL circuits, the op;- eration and function of which Will-fibe-described presently.

. A Once everyxother. .cycle of the alternating cur rent-frequency, whichis employedvto drive the tubesV |44 and |45; there is generated'animpulse in thewinding |64-of the transformer 54.-' '-.These impulses therefore occur. at 4 a raterrwhichsis substantially equal 'to twice the signal-i.reversal,lire-V quency; Vand,Y are impressed',uponconductors 51". .Thisyseries of impulses comprise the rst scanning-impulses previously referred to. and are .con-

A .nected to the-repeaterin vthe anodefcircuits of the mst-pick-up; tubes 50 and 5|:

The timing of these" impulses is suchA that the' received signals Which'are employed to condition theiirst'pick-up tubes are passed at thepropertimeto .the'storage tubes Sil'and'll.V` =There is 4also generated inthe secondary winding IE of the transformer |5358, series -of short impulses which::is.- 180i" outof phase with the series'. of: ilrstfsc'anning` impulses or, in terms of thetelegraplr signals,.idiffer in time byl one-half a baud. The impulses generatedfin the winding |65 are employed fora. purpose which will bedescribed morefully ina subsequent portion of the'description. Y

The alternating current potentialswhichare generated inthe secondary winding |661'of the transformer Mil (see Fiati-5) are connected` by meansof conductors |61 to thefinputcircuitsof a. pairof gaseous arc-discharge tubes |68 andi |69 `(see;f-l:1ie. 4).- The input and` output circuits of these tubesfaresubstantially similar to those pro; vided for theY tubes IMfand |45 of 3; Like: wise,-.,they `are operated at a frequencywhichfis twice that of` the highest :signal-'reversal frequency. -The transformer-'|10 is connectedfwith its lprimary winding |1| 1in-the-outputfcircuit` of thetube=l|68 so that vthere isgenerated inwthe secondary winding |12 a series 0f impulses having substantially `twice the signal reversal frequency. These` impulses 'are; connected toconductors:R4 10 and are employed to provide-spacecurrent for the second pick-up tubes 5ta-nd 61; These impulses arethe so-ealled secondiscanning-impulses prei .vously referred to-and control. theirtimingrzoff-the retransmitted signals.` There is connected to the, anode of the tube |69ethe vprimary windinga|`|3 of atransformer. |14, the secondarywvinding |15 of whichis utilized forl a testing purpose to be de-` scribed.

Thealternating current potentials Vof twice the signalreversal frequency. which appear in the output circuits of the tubes |68 and |891are .connected to the'terminals of a series arrangement of aV .pair of resistances |11 and |18. From these resistances are derived the input circuits of a pair of amplifier tubes |191and |80. These tubes may also be of. the 6127 type or their equivalents.v The output .circuits Lof these'tubes are connected to the terminals fof a primary windingf |8| of a trans` former :|32. Space current for the tubes is sup plied overconductorr|83 by the secondary winding |5fof the transformer |53: Thus, at a frequency. approximately equal to twice vthe signal reversal frequency, there, is completed a circuit for theiiow of space.y current in either or both ofthetubes |19 and 18B: If it is assumed that the idealconditions stillobtain, there will begener'- ated: inthe secondary winding ,|84 of the trans--4 former |82a seriesofvv transient potentials comprising-v one half cycle of onepolarity followedim'-` mediately by one half cycle` of the opposite polarity. These transient potentials are connectedvby means of conductors |85 to the frequency control circuits of Fig. 6 to which reference is now made.`

The impulses which arev generated in th'esccondary winding4 H52; in the ytransformer |60 are applied by means'of 'conductors |63 to the input circuit of an amplier tube |86, This tube may-be of the Eftype or its equivalent, The amplied impulses arev impressed upon the. primarywinding |31 cfaV coupling transformer |88; the secondary Windingflgof whichis shunted by a resistance |90 foit'the' purpose of regulating the amplitude of the impulsesinducedz'thereim The induced impulses, if they 'are of sufficient magnitude'are im-` pressed:l upon any integratingiqnetwork Alill by means i of a discharge device A| |12A lwhich may bea neon tube. If the* impulses are-generated under the-ideal' conditions assumed'the voltage thereof will beinsuicient tol breakdown1the neon tube |92.I It willbeiseen that this is of no material significance since the impulses, being; closely spaced vin timelwith respect "to'one another:and also. being; of :opposite tpolarity, 4would therefore cancel' one another if theywere admitted to the integrating network. Consequently, in the ideal situation assumed there is no corrective action exerted upon'the phasing frequency generator. y

Thereis asimilar arrangementl of apparatus to correct the transmitting frequency generator and includes-an amplifier tube |93, the-input of which is connected by means of conductors tothe secondary winding- |84. of the transformer |82. The-"amplified impulses are impressed` upon another integrating network |94 :by means of -a transformer ISE-and a neon tube |96.;

Beforefproceeding with ther-description of the operation of the apparatus described up vto -this pointunderactual operating conditions, it is believed that a theoretical consideration of the apparatus employed for altering thefrequencies of the two generators used in the system may leadto a better understanding of the operation of the specific apparatusfemployed to -control these free quenoieso For this purpose reference will be made to- Figi'?. 'There is shown schematicallyV an oscillator 4lill-which is provided with a primary source of.-energyffcomprisingfthertank T and two sec- Azet, Likewise, the one-unit battement 1.7,! ondary. sources of. energyccomprising;respectivelythe tanks TL and Tc.;'I;`he; primary -tank ;Tlis capablev of` sustaining; oscillations; Whenfacting alone at a: frequency which .isgdeterminely by.; the ,1 resonant. frequency ofthe condenser; I 9,8 andthe, inductance l 0,9, comprisnathis tanl` circuit;A The; secondary. tankv circuit Tnv also1 comprises a; con,- denser and an-.inductance andis;tuned;forireSQ nance at apoin-tfwhich isa ipredetermined -,anncunty above the resonant-frequency of the primary rilev T. Similarly, .the secondary'tankf'lo is.- t-llneol. 9.1"; resonance by. thesame. predetermined-:amount below the resonant frequency ofrthefplimary tanl The two secondari/tanks areincludedircsn-ectively.- inthe output circuits ofL vacuumtube s200;.andf Both of .tnesetnbesare excite.d;atthe-osci11af tor frequencyA bymeans of -coi1s.;2.0 2 and 20S-Which; are coupled to. the indllctancegwgofgthe primar-y tank circuit T. TlrierefareA alsoproyided in the, input. circuits. of. the tubes biasing arrangements whereby thetube imnedancesr-maybeivar-ied any desired manner; for. example thcfSwitches--Zllt and 205 in their upnen positions ,biastiie|.g1f ids -.o,fV the two tubes equally; It may'begseenthatuthis is accomplished byY connecting thetwo-lunitbat.- tery 205 between thearioliand cathode-of connected in seriesV witha-nother. -oneflunit battery 2,08 between the cathode; and;V grid of.` the tube; 2,01. When the switcneszo andaZBgare-plocedin their lower positions; the battery'-201 isconnected; in series with the batteryt. thereby.. fior,.rlisbirisl three units of biasingpotentialg toethetubefilll, Also,V Withtheswitchesinv their lower-positions, it may be, seen; that the tube 2 0] is biased-only by,-y the. potentiall of the oneunitfbattery 208; .'Ijhe, output circuits of, the tubes 2 0 0,and 20|are come, bined and coupled by means of.l resistances 2.0.51: and 2l0 to the output circuit of the-oscillntor; |91 Therefore, in additiongto theener-gywhichissupplied by the nrirnaryesource- T;,the oscillator isv furnished-. with energy from; the; two. secondary sources including the tanks Tnand Tot The operation., of theirequency control cin-icuits willbe described; with reference tOrf-Fiis; 83; and 9. Fig. 8 is a vectordiagramwhich shows, graphically the relations existing between 'sorneiof the voltages and currents:present ir1-.,thev circuits'. of Fig. '7-` at aI time whenfa state of, equilibrium, exists. In order tai-.simplify thedagrfmasmuch, as possible, it is assumed that only a;A fractional' part ci the output voltageof the oscillator iserninloyed to excite the vacuumtubes vZlifancl.- 201i... It is. further. assumed that this exciting.h .voltage when ampliedV by thev amplification; factor of the tubes is equal; to the output voltage QfLther oscillator; This voltage-maythen-be considered asrther driving voltage necessary toproduce av current. flow inthe output circuits of-the,vacuum-tubes;4 Consequently, this driving` voltage is represented in Fig. 8 asthevector Er andis. used as .the reference vector for all others shown in this dia. gram. Considering-first thefoutput circuit of@ the tube 200, it may be seen that., since thevtank-:Tn is tuned for res-onanceat,afrequencyhigher than,j the frequency of. thedrivingvoltage in the circuit,` it acts as an inductance. Consequentlythecure rent owing in theoutputcircuit of the .tubev is lagging with. respect tothedrivingivoltage.ET and is represented in thecdiagram. by. thevectorI-i. It is assumed that the resistance whlchis presentin the output circuit ot the tube 200 isrepresented bythe impedance of the tube. Obviously; otherresistances. may be `inserted inthe circuit without changing` the iundamental operating` character-1 Veltaee droloracroSs-thc tank anolls obpositeztofit,

in, phase; and.; is; representedby the vector LTL-i,

. The beitreten-on in the-.circuits obtainedbe addingthe-vectorsIiRT; and IlTgil andis repre- Sentediby the vector -ET Whichi it.- willf loe-ob.- scryeol,v iseqnal andonposite inpnasetothedriw ingvoltage ET.

A.. similaranalysis. can be made; offthe. output, circuit of.: the tube4 201|: in which. is,A included.; the. tank 'Ifo-which acts `as a-v capacity since itis,A tunedf for resonanceiavt a frequencyloelovvl that ofi; the ors;- cillator .|9J., Since -bothof the.Secondarytanks:y a-re:dctuncdby=ti1esameamount fromthe osov laton, .the effects, produced inl the, circuits are. e.clual,.v Therefore, thecorrcot lowinain,-the out?. out circuit. of; the.- tobo 2.01 maybereprosentedby the vector 12W-hien- Inay-be seen to lead theidriy ine voltage En by.- thc.. Same anale. as; .theY vector I1.: lags in: this i voltage. e ihe. reaction voltage. Iof .theY tube 201i is:represented by `the` vector 123m .andgis equal 1and opposite in.` phase to: .thevfoltase` drop; (not shown) acrossthe tube whichv is in.l phase.y withthefcurrent 12;;` Similarly, the, voltage 1 dropE E2 across the tank Tclags the. current Iz by an, anglexwhich approaches:.90: lIjhe reaction. Volt-.- age of" the `tank-'lc: is` represented by theevector 'IzTc which, is equal toEeand .opposite` in. phase. The sum. offthe .reaction.VoltagesIzRmand Iaic ,isi equal toY thed driving vvoltage andieoppositer threto-in phase and Ais represented bythe'vector.- v T.

- Sinceithesvoltages developedacross'the seconda-ry tanks are combined and added-to -thebutput voltage of` the oscillator, thisfeedfh-back Voltage mayY beA represented bythe vector Een which' ie thevectersum orthevoltages E1 and Itf may be seen that this food-back voltage isinv phaseV with the oscillator voltage Therefore, they are combined additively to producen,I voltage which may berepresented by thevector Eovvlvhich' is the voltage applied'l tothe load; circuit suppliedf by lthe oscillator.. Under these. conditions with," the feedrbaclvoltage phase with` the oscillatory output. .voltage estate ofconilbriuinexistsanolf selong as.l thev` circuit constants vare,,unc hanged anuV the. tubes. maintained.. with.' vequal bias'esoi their grids; theosoillator. will continnetooperate at axed frequency.` a

If thel switches .204: and 20,5[areplaced inthe-11;. lower positions t0,.incr;eas,e,I the positivebiason the tube 200 and simultaneously.v decrease the positivebias on the; tube 20L,- therefWiAll. b" produced. changes in.` theovutput/,circuit of theseqtubes, which will be analyzedv by. havingreference to Fig. 9. The driying Voltagevector Eri-fis` againA usedK for reference` Since the Vpositie/.e[bias of;- tb cf 200.1'1as, been. increased, .theimpedanceof` theinube; ishdecreased.. No'change is :made in thevignpedg-fance. ofy the` tan-vl; circuit, Tnrwhichacts as an doctance.. Conscooontlr this. element ot he .11i-- cuit exerts a.y eleaelA Qontrol; .A the content:ilovvinain.theol.1 --5. "e

the voltage and this current I'i now lags the voltage Er by a greater angle than in the previous case There is also an increase in the magnitude of the current because of the decreased total impedance of the circuit. The increase in the magnitude of the current and the decrease in the impedance of the tube combine to produce a tube reaction voltage which is slightly less than in the previous case, This reaction voltage is represented by the vector IiRT. Also, because of the increasev in the magnitude of the current, the reaction voltage of the tank T1. which is represented by the vector IiTi. is increased. It may be seen that the sum of these two reaction voltages is again equal and opposite in phase to the driving voltage Er. VIt will also be observed that the voltage drop across the tank TL represented by the vector E'i is also increased. In the circuit of the tube 2il1 the reverse conditions are present, that is, the current 12 is smaller and more nearly in phase with the voltage Er. Also, the tube reaction voltage IzR/T is slightly increased and changed in phase. In addition, the reaction voltage of the tank To is decreased as shown by the vector I2Tc. Again it will be observed that the summation of the vectors I'zRT and IzTo is equal'and opposite in phase to the voltage Er. Now it may be seen that the voltages E'i and E'z developed across the respective tanks Tr. and Tc when combined produce a feed-back voltage EFB which leads in phase with respect to the oscillator voltage ET. Consequently, the voltage which is supplied to produce oscillation is the vector sum of the voltages ET and EFB and is represented by the vector Eu. Also this voltage is seen to lead the voltage Er of the primary tank T and therefore the frequency of oscillation is increased.

While the unsymmetrical bias conditions are maintained on the tubes 200 and 2131, the oscillator frequency will increase until a second state of equilibrium is obtained. Since the tubes are being excited at the oscillator frequency, the higher the frequency becomes the more nearly the resonant frequency of the tank Tr. is approached and the greater the deviation from the resonant frequency of the tank Tc becomes. It will be obvious that, as the frequency increases, the current iiowing in the output circuit of the tube 2011 decreases and the current in the output circuit of the tube 201 increases. The reason for this is that the impedance of the tank TL increases with an increase in frequency while that of the tank-Tc decreases, It is evident that ultimately the frequency will reach a point at which a balance is restored between the two output circuits, at which point the currents owing in these circuits will again be equal in magnitude and will lag and lead, respectively, the driving voltage by the same angles. These are the conditions which are represented by the vector diagram of Fig. 8 and it has been demonstrated that the feed-back voltage under these conditions is in` phase With the oscillator voltage and stable conditions exist.

It is believed that it will be obvious to those skilled in the art that an increase in the positive biasing of the tube 201 and a corresponding decrease in the positive biasing of the tube 21l0-will be effective to produce a feed-back voltage which is lagging with respect to the oscillator voltage. This condition will result in a decrease in the oscillator Vfrequency until a new frequency is reached at which stable conditions again will eX- ist. i' Thus it is evident that such a system provides analmost instantaneous effect upon the oscil- .2() lator in response to a change in the biasing of the vacuum tubes employed.

Having reference again to Figs. 5 and 6, there is shown a vacuum tube 211 which is provided with an input circuit connected between its grid and cathode which includes a portion of a potentiometer 212 and the integrating network 191. The potentiometer 212 is connected across the terminals of a secondary winding 213 of the transformer S6. Consequently, the tube 211 is driven at a frequency which is equal to that of the oscillator 85. The integrating network l91 serves as Ythe means for altering the bias applied to the grid of the tube 211 in a manner which will be disclosed presently. The output circuit of this tube is connected across the terminals of the battery 82 and includes a resistance 214, a tank circuit 215, and the space discharge path of the tube 211. The resistance 214 is shunted'by a condenser 216, the purpose of which is to bypass the high frequency currents around the resist-l ance. There is also connected across the terminals of the battery 82 a voltage divider including the resistance 214 and resistances 211 and 218 and a portion of a potentiometer 219 which isv connected to another secondary winding 22%` o the transformer 96.

There is also provided in the phasing frequency corrector another vacuum tube 221 which is provided with an input circuit connected between its grid and cathode and which includes the resistance 218 and a portion of the potentiometer 219. The output circuit of this tube is connected across the terminals of the battery 82 and includes a resistance 222, a tank circuit 223 and the space discharge path of the tube. The resistance 222 is shunted by a condenser 224 for the purpose of providing a low impedance path for the high frequency currents.

The tank circuit 215 constitutes a secondary tank circuit which is tuned for resonance at a frequency which is higher than that of the primary tank circuit 85-8'1 of the oscillator Se. The tank circuit 223 forms the other secondary tank circuit and is tuned for resonance at a frequency which is lower than that of the primary tank circuit associated with the oscillator Bil. Hence, as far as the alternating currents are concerned the tank circuit 215 acts as an inductance and the tank circuit 223 acts as a capacity. These two secondary tank circuits are connected respectively through resistances 225 and 226 and are connected through a direct current blocking condenser 221 to the primary tank circuit :i6-Sl.

The correcting action of the phasing frequency corrector circuits outlined above will be described by assuming that conditions of equilibrium obtain and the oscillator Si! is operating at a given frequency. If now one or more correcting impulses are impressed upon the integrating network 191 from the transformer of Fig. 3 and these impulses are of the same polarity, the grid bias of the tube 211 is changed. If the impulses are of such a nature that the grid of the tube 211 is more positively biased, the tube will pass more current. An increase in the current flowing in the plate circuit of the tube 211 decreases the positive potential of the point 228. Since this point is connected through the resistance 211 to the grid of the tube 221, the grid of this tube is made less positive with respectto its cathode, thereby reducing the current flowing in its output circuit. The increase of currentflowing in the inductive tank circuit 215 and the decrease of the current owing in the capacitative tank circuit 223 with the attendant phase shifts-of these currents with respect to the driving voltage results in the feed-back to the primary tank circuit 86-81 of the oscillator 80 of energy leading in phase with respect tothe primary tank circuit energy to produce an increase in the frequency of the'alternating current generated by the oscillator-in accordance with the theory of operation outlined hereinbefore. A`s previously described, this increase in frequency produces changes in the phasing frequency corrector circuits of such a nature that equilibrium is again established to maintain the frequency at this new point.

"Obviously an increase in the `frequency generated by the oscillator 8|) will produce an increase in the phasing frequency which is derived from the combination of the outputs of oscillators 80 and 91 because the frequency of the latter generator remains constant.

'I'he correcting impulses which are to be used to vcontrol the frequency ofthe transmitting frequencygenerator are applied to the integrating network |94. This network serves to vary the biasof a-vacuum tube 229. The input circuit for this tube includes a portion of the potentiometer 230 which -is connected to a secondary winding 23| of the transformer |69 Aand also the integrating network |64. The output circuit of this tube is connected across `the terminals of the battery 82 and includes a resistance 232, a secondary tank circuit 233 which is arranged to act as an inductance and the space discharge path of the tube. The resistance 232 is shunted by a condenser 234 for the purpose of providing a low impedance path for the high frequency currents. A tube 235 is also included in the transmitting frequency corrector and is provided with an input circuit including a portion of the potentiometer 236 which is connected across another secondary winding 231 of the transformer |09. The output 4circuit of this tube includes a resistance 238 connected in series with a secondary tank circuit 2'39 which is arranged to act asa capacity. The resistance 238 is shunted by a condenser 249 to bypass the high frequency currents.

The impedance of the tube 229 is altered in accordance with the correcting impulses which are applied to the integrating network |94 by the transformer |62. The grid of the tube 235 is oppositely biased in a manner similar to the biasing of the tube 22|. The energy components derived from the secondary tank circuits '233 and 239 are combined and connected through the direct current blocking condenser 24| to the primary tankcircuit ||2 associated with the oscillator |64. The control of the frequency of this oscillator is effected in substantially the same manner as that of the oscillator 86. Consequently, any change in the frequency of the oscillator |94 results in a change in the frequency of the transmitting frequency generator.

So long as the signals which are being received from the line W are not subjected to influences which produce phase shifts, the phasing frequency generator and the transmitting frequency generator are maintained constant both as to frequency and their mutual phase relationship. However, such ideal conditions are not easily realizable in practice. Therefore, the operation of the operation of the repeater will be described under conditions which are met in operating the system over line wires which are subject to interference.

For the consideration of one phase of the operation of the repeater. assume that the periodic 22 devicewhich is employed to time thetransmission ofthe signals from the remote station-connected to the line W is maintained constant. Under these conditions, the onlyy phase shifts which may be present in the signals 4that are lreceived from this line by the apparatus herein disclosed are those which areY produced by interference effects, such as -the introduction-into the signals of extraneous currents. Suppose Lthat the arrival time cfa given signal is '-laterthan itsimmediate predecessor, which was a perfectly timed signal. At the time of -thereversal produced bythe latearriving signal, an impulse is generated in the secondary winding -|6| ofthe transformer 29. This impulse 'isemployed by the phase comparator comprising the tubes associated circuits 'to `generate a correcting impulse in the secondary winding |62 of the transformer |60. Since the `impulse generated inthe winding |6|' occurs at a time whi-ch is later-than normally, the phase comparator tube |51 will -be conditioned -for lthe conduction of space current. The amplitude of the correcting impulse -is sufcient, when amplified, to break down the neon lamp |92. The polarity v'of the correcting `irnpulse generated in .the transformer winding |62 is such that an impulse of applied through the neon lamp |92 to the integrating network 9|. As previously described, such a condition tends 4to lproducea change in the frequency of oscillation of the oscillator 80 of such ia nature that the frequency of the -phasing frequency generator tends tofbe decreased. In actual practice, the system is ybest `arranged to require a succession cfa-small Anumber of like impulses to `be generated before a change in the frequency of the generator is produced. However, it is thought that the roperation may -be better understood if it is assumed that this change in frequency is made substantially instantaneously so that the next half cycle ofthe phasing frequency is slightly delayed. When it does occur, the tube is rendered conducting and the surge of space current through the winding |52 of the transformer |54 Agenerates the first ,scanningA impulse in thewinding |64.- 'This impulse is Vgenerated at a ltime which is approximately one-half a baud, or signal impulse, .later than the arrival time of the signal. In accordance .with the polarity of the received signal, the first scanning impulse which 1is impressed by conductors 51 upon the anode circuits of .the rstpickup tubes and 5| results in the operation of one of the storing tubes 59 or 60. At a time, equal to the time .delay of the ,received signal less than one-half a baud, after the operation of the storing tubes, the 'signal is passed to the transmitting tubes 12 and 13` for transmission over the line E. In other words, there is no changemade in the spacing of the retransmitted signals, even though a change has occurred in the received signals and a corresponding change has been made r in the phasing frequency generator.

One-half cycle of the phasing frequency later than the generation of the rst scanning impulse, the tube |44 is rendered `conducting to generate in the winding |65 kof the transformer |53 a comparison impulse which is employed by the phase comparator includ-ing tubes 19 and |89 for the purpose of comparing the phase relation-s `of the phasing frequency and transmitting frequency generators. `Since a slight change has been madey in the frequency of the vphasingfrequencygenerator prior 4to the comparison, asmall |51 and |58 and their.

negative polarity is' sals and are phase difference will be detected and a correcting impulse generated in the secondary winding its of the transformer KS2; This correcting impulse is utilized to apply an impulse to the integrating network 84 of such g-polarity (in this case negative) that a tendency is produced to decrease the frequency of the transmitting frequency generator. However, the integrating network is arranged so that one such impulse is virtually ineffective to produce any change in the frequency of the transmitting frequency generator. Consequently, when one late-arriving signal is retransmitted over the line E, there is no difference in the timing interval between it and its predecessor than in any of the previously transmitted signals.

The reason for such an arrangement will be apparent when it is considered that the arrival time of the following signal may be early with respect to the signal just considered, or nearly even with respect to a perfectly timed signal. In either case, a discrepancy between the phase of the early arriving signal and the phasing frequency generator will be detected by the phase comparator including the tubes 151 and 158 and an appropriate correcting impulse applied to the phasing frequency corrector to increase the frequency of the phasing frequency generator. Thus, it is seen that so long as the frequency of the originally transmitted signals is unchanged, phase differences occurring in the individual signals as they are received at the repeater are continually operating to correct the phasing frequency generator so that it may be kept in step with each individual signal. At the same time, substantially no change is made in the frequency of the transmitting frequency generator, with the result that the retransmitted signals are unchanged as to their spacing from one another in point of time.

Suppose now that the frequency of the originally transmitted signals begins to drift in one sense or the other, say the tendency is for the frequency to decrease. Then, aside from the effects of fortuitous disturbances on the received signals, each signal will arrive later than its predecessor. Hence, the operation of the phasing frequency corrector effects a decrease in the frequency of the phasing frequency generator. Consequently, there is a. succession of like correcting impulses generated. in the winding |84 of the transformer H32, which produces an accumulation of impulses in the integrating network ld which, after a period of time, becomes effective to alter the frequency quency generator. This change, however, is of such a gradual nature that a slow change is made in the frequency of the retransmitted signals. Therefore, there is a minimum of distortion of a phase shifting character produced in the retransmitted signals. Y

In operating a synchronous telegraph system of the multiplex type, it is the practice at times when no signals are being transmitted to periodically reverse the polarity of the potential which is applied to the line circuit. These reversals occur at a frequency which is substantially less than that of the highest telegraph signal reverfor the purpose of maintaining synchronism between the f terminal apparatus. These synchronizing reversals are also effective to maintain synchronism between the terminal apparatus and the equipment of any repeaters included between the two terminals which employ periodic timing mechanisms. One such repeater of the transmitting frez 24 is that in which Vthe present invention is embodied. Y

Instead of the relatively low frequency synchronizing alternating current which is transmitted, it is necessary at other times to transmit alternating current signals having a frequency substantially equal to the telegraph signaling frequency. For example, when the system is being lined up, it is the practice for each repeater station to transmit alternating current at signaling frequency to the next repeater station or to a terminal station in order that the attendant at the remote station may balance his artificial line while he is receiving at his station the signal frequency alternating current. It sometimes becomes necessary to make such a line-up at times when telegraph signals are being received from the opposite end of the circuit; In any event, it is necessary to prevent either the telegraph signals or the synchronizing reversals being received from the opposite direction from being transmitted to the station at which the line-up is being made.

In the regenerative repeater disclosed herein there are two sources of alternating current, but each has a frequency which is twice that of the highest signal reversal frequency. In order to utilize the alternating current derived from one of these local souces, it thus becomes necessary to halve the generated frequency in order to transmit alternating current at the telegraph signaling frequency. This is accomplished by utilizing the operating characteristics of the transmitting apparatus including the gas discharge tubes and the delayed action positive feedback circuits with which these tubes are provided.

Having reference again to Fig. Ll, the manner in which one of the local sources of oscillations is employed to operate the transmitting tubes l2 and 'i3 will be described. When it is desired to transmit to the remote station connected to the line E alternating current at signalling frequency, a switch 2&12 is moved to the left. It may be seen that by this operation the secondary winding 'Il of transformer 69 is short-circuited, thereby preventing any of the signals received from the line W from affecting the operation of the transmitting tubes and at the same time the input circuits of the transmitting tubes are connected in multiple. Also, the connection from the mid-point of the secondary winding 'il to the common cathode circuit is connected to include the secondary winding |15 of the transformer H5. There is induced in the winding |15 a series of unidirectional impulses at a frequency equal to twice the -telegraph signaling frequency. The winding is connected so that this series of impulses periodically renders the grids of both of the transmitting tubes 'l2 and 13 positive with respect to their associated cathodes.

The operation of the transmitting tubes to transmit signal frequency alternating current to the line'E will be described by assuming that the tubelZ is conductive and the tube 'i3 is non-conductive. The space current of the tube 'l2 owing lthrough the common cathode resistance applies a fixed negative bias to the grid of the tube i3 as described. Also, a predetermined time after the extinction of the arc in the tube 13 a positive bias is also impressed upon the grid of this tube `"by means of the delayed action network. In accordance with the foregoing description of these circuits this time is approximately equivalent to one-half of the baud time. Both of the attachee.

transmittingV tubesK arecprovided. with .such .-networks-,.-the-.o-peration of which has .been described previously.'i Asdescribed, the combination'of the negatives andipositive. biases is insuicient .to operate the tubes. pulses .isgenerated in the secondary., winding offthe .transformer Hit; the. transmittingtube ll! remains...unoperated.` However, vwhen the neXt impulseis generated bythe transformer l'lll""and Therefore, until one oftheim- 5 positive potential-with' respect to the cathodes is 10 applied. to the grids .of "the `transmitting tubes, the tube'l'Z'which isalready'operatedlis unaffected, butA a discharge is initiated 'in-.the unoperated tube.l3 YIt should'be'notedthatthe impulses .which are applied'to the input circuits of l5 these tubes are of'relatively'sh'ort duration; some# Whatl'ess than one-half "a .signal baud'. Conse# quently, it is after the termination ofthe impulse that the delaynetwork operates'to'apply positive feed-'back potential to the 'tubelZ 'which 'was eX-' tinguished by the operation 'of the'tube'73.v However, it is important to. understand that th'e1mag-' nitude of the' impulse is insufficient'toreLop'erate the tube 'l2 withoutth'e aidof the positive'feed'- back potential. Therefore;so long'as the time delay introduced bythe networlf'is greater than the time duration of' the impulse, onlyone tube. will be operated by one impulse.'

Thus; it is' seen' that'th-e succession of `unidirectional impulses appliedV simultaneously to the input circuits "ofthe Vtransmitting tubes produces the alternate operation" of theseV two' tubes; Therefore, the 'signals' which' appear' in the input circuits'of'the transmitting tubes andwhich'are transmitted over.' the line E"'have arfrequency 35 which is exactly oneeh'alf. offthatof the impulses impressed'upon'the'input circuits of the'tubes.

The foregoing description ofthe'frequency di'- vi'der' circuits' employedinithe regenerative repeater"is'concerned onlyjwith' the fundamental 40 or essential elements.' Reference has been' made to" a` series ofV unidirectional*impulses employed to operate'the transmitting'tubes 'l2 andl in the'manner'described. Actually thereare"applied to the" multipled' input circui'tslof thesetub'es other potentials which are'in'aictive as far'as these circuits are4v concerned; and"therefore 'have no ef'` fect uponth'eoperation ofthe transmitting tubes; Theseotherpotentials are unidirectional impulses Ofopposite' polarity to thoseutiliz'e'cl' by the tubes and are spaced' 180 electrical degreestherefrom'.

It Will be apparent that the-inactive-,as4 well asthe active', series `of unidirectional impulses isderived fromH the" secondary winding |15" of thetrans'- formercll'when it is considered-that the tubes l58 andv H59 are operated alternately and instan-y taneously in' response to a'sinusoidal alternating current potential.` Therefore; there' is' produced iny 'the' output' circuitsofthese 'tubes'4 a substantially square-toppedalternating currentf At'the beginningofieach ofthe alternatehalf-cycles of 60 the output alternating 'current' there is* produced b'yinduction in'the secondary winding l'l5'of the transformer IleA a transient'volta/ge"of onepolarity' and ofu relatively short duration; Also, at

thel endof each of'these alternate' half-cycles f there is'produced in'the'transform'er secondary Windinga" transient voltage'foftheopposite po# larity` and of relatively short' duration. The

transient' voltages developed inthe winding` |75 70 are impressed yupon the multipled Vinput' circuits of'thetransmitting tubes ll2and I3 It Will be obvious to those skilled in the'art'that'only those impulseswhich render-thegrids positive withree speci: to their associated"cathodesf'are enective'in 75 26 these circuits. The Aother series of impulses which render 'the grids negativewith respect to the-cathodes does not'anectthe operation-of the `transmitting'tubes inanyway;V

Thus, it should be apparent that `the-impression' of alternating currentimpulses of the character described upon the input 'circuits' of thetransmitting tubes connected in parallel will result inthe production in the output circuits of these tubes ofV an alternating current having a frequency which is the second'subharmonic of the-alternating current applied to the input circuits.

The frequency divider `connected'anol adjusted as described' utilizes a series'of relatively short im pulses'for its successful operation. If, instead' of adjusting the delay networks connected to'these tubes so that the positive feed-back potentialslare applied' tothe gridsof the tubes with a timeidelay equal substantially to one-half'a' baud',VA they are adjusted tok producers; tim'e"delayequal to slightly morethan'on'ehalf 'a baud, sayl -threeequarters of a' baud,'each of the impulses may comprise-acomplete half cycle of the alternating currentapplied tothe input circuits of the tubes. Such an ar rangement will permitthe 'operation of the trans mitting tubes "l2and 'I3r in response to an alternating current input having substantially any constant waveform to produce in the output circuit ofthel tubes an alternatinglcurrent having a frequency which is exactly half-of the `frequency ofthe input 'alternating current. Such an arrangement wouldlv permitA the elimination-ofthe tubes l 68 and |69 and their associated apparatus.A In this case;- the alternating current'which is Vapplied to the grids of these tubes-by means of con'- du'ctors l 6l would be utilized in placeof the impulses derived fromthe secondary 'wind-ing l'lof transformer" I'li ItY will be apparent to those-skilled in' the' art that, by suitably adjusting the delay networks to produce a time delay in the impression of the positivefeed-back potential equal to slightly less than the time required for one-half cycle of the desired alternating current; other sub-multiples-of an alternating current may-befobtained. ForeXam-ple, the delay can be made equal to slightly less than two bauds. In this case, the' first' positive impulse operates on'e lof thetransmitting tubes,v say the tube 13. The second-positive impulse which is spaced in time one baud from-'the rst impulse is ineiective since the4 positive feed-back potential is absent at the time. However, this biasing' potential is present by the time' that the third posi'- tive impulse is impressed uponthe input-circuits ofthe tubes and causes the operation of the tube 72'.' Thus it' isfseen that' oncef every 'fourth' half cycle of the? input alternating current; thereis produced' one half cycle of the output alternating current; or'in other words, the input alternating current isdivided by four.

The frequency divider'di'sclosed' herein is' sus: ceptible of use with any thermioni-cdevice' ar-v rangedto operate asa trigger fdevi'ce.' Such' operation is. an inherent" characteristic ofthe. gaseous arcf discharge 'tubes'l employedin Ithe vpresent 1re-'- peater. However, other thermionic tubes in whichithe: anode' current is at' all'times under the control of thev grid" element maybe'arranged to' operate as trigger tubesfb'y connecting them in accordance' with'the instant disclosure; For example, the two tubes 59 and'60`of Fig. 3 connected f as shown with inverse-biasing circuits may also be provided withthe delayed action positive feed-back circuits with which' the. gas-` flledf tubes:shown-herein-` are providecl;A When menace 27 thermicnic vacuum tubes, preferably tubes having high amplification factors, are connected in this manner, their operation in response to critical potentials is substantially similar to the operation of gaseous arc discharge tubes. In this case, it may be desirable to suppress those impulses which render the grids negative with respect to the cathodes, such as by shunting the transformer winding 115 with a suitably poled rectifier. Therefore, it is apparent that the frequency divider apparatus shown and described in detail is not necessarilyV limited to the actual apparatus and connections shown. Instead, it is susceptible of a much broader use in accordance with the foregoing outline of other modifications thereof. v Y

The repeater circuits, described herein may be duplicated and connected between the lines E and W in the reverse manner to those disclosed in order that westbound signals traversing theA circuit between the two remote stations may be regenerated also. In that case, the westbound receiver 243 is connected as shown to the terminating impedance 18 associated with the line E. Connections are made from the receiver 243 to the westbound regenerator 244 and thence to the westbound transmitter 45. This transmitter, in turn is connected to the apex of the terminating impedance 24 associated with the line W. Both eastbound and westbound repeaters function independently so that full duplex operation of the system is effected.

While the invention has been disclosed in what, at present, is considered a preferred embodiment, it is not contemplated that the invention be limited to the specific instrumentalities illustrated. For example, the types of tubes employed herein have been designated so as to enable one skilled in the art to practice the invention. However, it is obvious that other types of tubes may be employed, together with correspondingly different values of impedances, potentials, and so forth, without departing from the spirit of the invention. Accordingly, the scope of the invention is defined in the appended claims.

What is claimed is:

1. In combination, an electronic oscillator, a source of primary periodic energy to determine the normal frequency of said oscillator, two auxiliary resonant generators respectively producing periodic energies one of which leads and the other of which lags the phase of said primary periodic energy by equal angles each of less than one-half wave length relative to said primary energy, means for combining said auxiliary periodic energies to supply supplementary energy to said oscillator in phase with said primary periodic energy, and means for oppositely changing the phase angles of said auxiliary energies to displace in phase said supplementary energy with respect to said primaryenergy.

2. In a frequency control system, a primary generator of wave energy, two secondary resonant generators of wave energies, one leading and the `other lagging the phase of said primary wave energy by less than one-half wave length of said primary-energy, a utilization circuit, means for combining said secondary energies with said primary energy to supply said utilization circuit, the combination of said secondary energies being normally in phase with said primary energy, means for exciting said secondary generators fromy said utilization circuit, means for oppositely varying the phase of said secondary wave energiesto cause a succession of like phase variations of the energy supplied to said utilization circuit, and means for causing the combination of said secondary energies to be restored in phase with said primary energy and thereby stabilize the system at a new frequency.

3. In a system for varying the frequency of a generator between predetermined limits, an oscillation generator including an electron discharge tube having input and output circuits, a first impedance which increases with frequency within said limits coupling the input and output circuits, a second impedance which decreaseswith frequency within said limits connected in multiple with said first impedance, two control circuits each including one of said impedances, an impedance which is independent of frequency included in each of said control circuits, means for causing alternating currents derived from said generator to flow in said control circuits, and frequency-varying means to oppositely alter said frequency-independent impedances.

4. In a frequency control system, an oscillation generator including an electron discharge tube having inputand output circuits, a reactive network having an inductive characteristic at the normal generator frequency, another reactive network having a capacitive characteristic at the normal generator frequency, the total impedance of said networks having a substantially constant value throughout a range of frequencies varying a predetermined amount above and below the normal generator frequency, two parallel circuits each including one of said networksto couple said input and output circuits and thereby furnish energy to said generator, means for causing alternating currents derived from said generator to flow in said networks, and means for varying said currents to vary the frequency of said generator by causing a phase shift of the energy furnished to said generator by saidparallel circuits.

5. In a frequency control system, an oscillation generator including an electron discharge tube having input and output circuits, a primary tank circuit comprising inductance and capacitance tuned for resonance at a normal frequency coupling said input and output circuits, an inductive secondary tank circuit comprising inductance and capacitance tuned for resonance at a frequency which is a predetermined amount higher than Said normal frequency, a capacitive secondary tank Vcircuit comprising inductance and capacitance tuned for resonance at a frequency which is said predetermined amount lower than said normal frequency, said secondary tank circuits each being connected in multiple with said primary tank circuit, means for causing alternating currents derived from said generator to flow in said secondary tank circuits, the current in the respective circuits normally being of equal magnitude, and means for varying the relative magnitudes of said currents.

6. In a frequency control system, an oscillation generator including an electron discharge tube having input and output circuits, two auxiliary circuits including respectively inductive and capacitive reactance elements, two feed-back circuits each including one of said reactance elements coupling said output circuits, means for normally causing alternating currents having the frequency of said generator to flow in said auxiliary circuits, said Vcurrents being normally equal and thereby producing in said feed-back circuits equal energies which differ in phase by equal but opposite angles with respect to said generator, and means for unbalancing said alternating cur- 

